Privileged tool call following untrusted retrieval (indirect prompt injection trail)

Detects the canonical indirect-prompt-injection trace shape: an untrusted RETRIEVER span (e.g., web fetch, user-uploaded document, third-party API result) is followed in the same trace by a TOOL span whose privilege is write, destructive, or exfiltrative. This is a trace-method rule using the `forbid` primitive (spec/atr-method-v1.1.md §8.3.1) with a `preceded_by` predicate. Threat model: indirect prompt injection works by getting a malicious instruction into content the agent retrieves (not into the user's prompt). The agent reads the retrieved content as data but executes embedded instructions as if they came from the user. The signature in the trace is unmistakable — an untrusted retrieval span immediately feeds into a privileged tool call that does not match the user's original request. AgentDojo (arXiv:2406.13352) catalogs 629 cases of this pattern; InjecAgent (arXiv:2403.02691) ships 1,054. ATR encodes the shape, not any single instance. Pattern detection (regex on agent.output text) cannot reliably catch this because the malicious payload can be paraphrased, encoded, or natural-language-only. The trace shape — untrusted source → privileged effect — is invariant.

建議回應

參考資料

1. 01

   Synthetic field emitted by the trace engine when the forbid primitive fires; trace-native engines evaluate detection.trace.

   欄位: trace.forbid_violation運算子: regex

攻擊範例（規則會觸發）

1. {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://attacker.example.com/payload"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"email.send","tool.privilege":"exfil","args":{"to":"[email protected]"}}}]}
   

2. {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"l1","kind":"LLM","attributes":{}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}}]}
   

3. {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://docs.public/page1"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"db.delete","tool.privilege":"destructive"}}]}
   

4. {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"r2","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"data.exfil","tool.privilege":"exfil"}}]}
   

5. {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://attacker/index"}},{"id":"a1","kind":"AGENT","attributes":{"agent.name":"summarizer"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"slack.post_to_channel","tool.privilege":"write"}}]}
   

以上為真實攻擊 payload 脫敏版本。用於 regression testing。

正常樣本（規則不會觸發）

1. Retrieval source is trusted — no untrusted precursor

   {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"trusted","source.url":"https://internal.corp/kb"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"email.send","tool.privilege":"exfil"}}]}
   

2. Untrusted source but tool privilege is read — no privileged effect

   {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"calculator","tool.privilege":"read"}}]}
   

3. Privileged tool with no RETRIEVER predecessor — direct user-initiated, not injected

   {"spans":[{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}}]}
   

4. Untrusted RETRIEVER appears AFTER the privileged tool — wrong causal direction

   {"spans":[{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}},{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}}]}
   

5. All retrievals trusted — exfil tool used legitimately (e.g., authorized data export)

   {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"r2","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"data.exfil","tool.privilege":"exfil"}}]}
   

已知誤報情境

• ▸Parallel sub-agent FP: two unrelated sub-agents share one trace; one retrieves untrusted content for task A while a second performs a legitimate privileged write for task B. The `preceded_by` semantics in spec §8.3.1 is temporal-existence, not causal data-flow. Mitigation: engines that emit causal edges (e.g., parent-span links) SHOULD scope `within_trace` to the causal subgraph rather than the full trace DAG. Until then, this rule has a non-zero parallel-sub- agent FP rate; treat wild_fp_rate=0 as approximate for single-agent traces only.
• ▸Trust-label tampering bypass: an attacker who controls the retriever's instrumentation may emit source.trust="trusted" on malicious content. The rule cannot defend against trust-label forgery; operators MUST verify trust labels are emitted by a trustworthy span emitter (typically the agent platform, not the retrieved content's author).
• ▸Authorized exfiltration FP: data-export tools (e.g., scheduled backup, compliance report generation) carry tool.privilege="exfil" legitimately. If they follow any untrusted retrieval in the same trace, the rule fires. Mitigation: tag scheduled/authorized exfil spans with attributes.policy_exemption="export-job" and run a pre-filter in the engine policy layer.

title: "Privileged tool call following untrusted retrieval (indirect prompt injection trail)"
id: ATR-2026-00550
rule_version: 1
status: draft
description: >
  Detects the canonical indirect-prompt-injection trace shape: an
  untrusted RETRIEVER span (e.g., web fetch, user-uploaded document,
  third-party API result) is followed in the same trace by a TOOL span
  whose privilege is write, destructive, or exfiltrative. This is a
  trace-method rule using the `forbid` primitive (spec/atr-method-v1.1.md
  §8.3.1) with a `preceded_by` predicate.

  Threat model: indirect prompt injection works by getting a malicious
  instruction into content the agent retrieves (not into the user's
  prompt). The agent reads the retrieved content as data but executes
  embedded instructions as if they came from the user. The signature in
  the trace is unmistakable — an untrusted retrieval span immediately
  feeds into a privileged tool call that does not match the user's
  original request. AgentDojo (arXiv:2406.13352) catalogs 629 cases of
  this pattern; InjecAgent (arXiv:2403.02691) ships 1,054. ATR encodes
  the shape, not any single instance.

  Pattern detection (regex on agent.output text) cannot reliably catch
  this because the malicious payload can be paraphrased, encoded, or
  natural-language-only. The trace shape — untrusted source → privileged
  effect — is invariant.
author: "ATR Community"
date: "2026/05/28"
schema_version: "1.0"
maturity: draft
severity: critical

references:
  owasp_llm:
    - "LLM01:2025 - Prompt Injection"
    - "LLM06:2025 - Excessive Agency"
  owasp_agentic:
    - "ASI01:2026 - Prompt Injection (Direct + Indirect)"
    - "ASI04:2026 - Unauthorized Resource Access"
  mitre_atlas:
    - "AML.T0051.001 - Indirect Prompt Injection"
  research:
    - "AgentDojo: A Dynamic Environment to Evaluate Prompt Injection Attacks (arXiv:2406.13352)"
    - "InjecAgent: Benchmarking Indirect Prompt Injections in Tool-Integrated LLM Agents (arXiv:2403.02691)"
    - "ICON: Indirect Prompt Injection Detection via Focus Intensity (arXiv:2602.20708)"

compliance:
  nist_csf:
    - "DE.CM-09"
    - "PR.AC-04"
  etsi_ts_104223:
    - "P4.4"
  eu_ai_act:
    - article: "15"
      context: >
        Cybersecurity — indirect prompt injection via untrusted retrieval
        into privileged tool calls is the canonical agentic attack chain
        under Article 15.
      strength: primary
  nist_ai_rmf:
    - subcategory: "MS.2.6"
      context: >
        Information security — untrusted-input-to-privileged-action chains
        in agent traces are integrity violations requiring detection per
        MS-2.6.
      strength: primary

tags:
  category: prompt-injection
  subcategory: indirect-injection-trace-shape
  scan_target: runtime
  confidence: high
  source: trace-rule

agent_source:
  type: agent_trace
  framework:
    - any
  provider:
    - any

detection:
  method: trace
  condition: any
  false_positives:
    - >
      Parallel sub-agent FP: two unrelated sub-agents share one trace; one
      retrieves untrusted content for task A while a second performs a
      legitimate privileged write for task B. The `preceded_by` semantics
      in spec §8.3.1 is temporal-existence, not causal data-flow.
      Mitigation: engines that emit causal edges (e.g., parent-span links)
      SHOULD scope `within_trace` to the causal subgraph rather than the
      full trace DAG. Until then, this rule has a non-zero parallel-sub-
      agent FP rate; treat wild_fp_rate=0 as approximate for single-agent
      traces only.
    - >
      Trust-label tampering bypass: an attacker who controls the
      retriever's instrumentation may emit source.trust="trusted" on
      malicious content. The rule cannot defend against trust-label
      forgery; operators MUST verify trust labels are emitted by a
      trustworthy span emitter (typically the agent platform, not the
      retrieved content's author).
    - >
      Authorized exfiltration FP: data-export tools (e.g., scheduled
      backup, compliance report generation) carry tool.privilege="exfil"
      legitimately. If they follow any untrusted retrieval in the same
      trace, the rule fires. Mitigation: tag scheduled/authorized exfil
      spans with attributes.policy_exemption="export-job" and run a
      pre-filter in the engine policy layer.
  conditions:
    - field: trace.forbid_violation
      operator: regex
      value: "(?i)forbid_violation:untrusted_retrieval_to_privileged_tool"
      description: >
        Synthetic field emitted by the trace engine when the forbid
        primitive fires; trace-native engines evaluate detection.trace.
  trace:
    ingest_format: openinference
    forbid:
      - shape:
          span.kind: "TOOL"
          attributes:
            tool.privilege:
              in: ["write", "destructive", "exfil"]
        preceded_by:
          span.kind: "RETRIEVER"
          attributes:
            source.trust: "untrusted"
        within_trace: true
        description: >
          A privileged TOOL span (write/destructive/exfil) MUST NOT be
          preceded in the same trace by a RETRIEVER span whose source is
          marked untrusted. The shape captures the indirect-prompt-
          injection causal chain regardless of payload content.

response:
  actions:
    - block_tool
    - quarantine_session
    - alert
    - snapshot
  message_template: >
    [ATR-2026-00550] CRITICAL: Indirect prompt injection trace shape
    detected. Untrusted RETRIEVER span {{trace.preceded_by_span.id}}
    (source: {{trace.preceded_by_span.attributes.source.url}}) was
    followed by privileged TOOL span {{trace.matched_span.id}}
    ({{trace.matched_span.attributes.tool.name}}, privilege=
    {{trace.matched_span.attributes.tool.privilege}}). Block the tool,
    snapshot the full trace, quarantine the session, and audit the
    retrieved content for embedded instructions.

confidence: 88

wild_fp_rate: 0

test_cases:
  true_positives:
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://attacker.example.com/payload"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"email.send","tool.privilege":"exfil","args":{"to":"[email protected]"}}}]}
      expected: triggered
      description: "Untrusted retrieval immediately followed by exfiltrative email tool"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"l1","kind":"LLM","attributes":{}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}}]}
      expected: triggered
      description: "Untrusted retrieval, then LLM, then privileged write — chain still triggers"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://docs.public/page1"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"db.delete","tool.privilege":"destructive"}}]}
      expected: triggered
      description: "Untrusted retrieval followed by destructive DB tool"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"r2","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"data.exfil","tool.privilege":"exfil"}}]}
      expected: triggered
      description: "Mixed retrievals — at least one untrusted before privileged tool still triggers"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted","source.url":"https://attacker/index"}},{"id":"a1","kind":"AGENT","attributes":{"agent.name":"summarizer"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"slack.post_to_channel","tool.privilege":"write"}}]}
      expected: triggered
      description: "Untrusted source, intermediate agent reasoning, then privileged Slack write"

  true_negatives:
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"trusted","source.url":"https://internal.corp/kb"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"email.send","tool.privilege":"exfil"}}]}
      expected: not_triggered
      description: "Retrieval source is trusted — no untrusted precursor"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"calculator","tool.privilege":"read"}}]}
      expected: not_triggered
      description: "Untrusted source but tool privilege is read — no privileged effect"
    - input: |
        {"spans":[{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}}]}
      expected: not_triggered
      description: "Privileged tool with no RETRIEVER predecessor — direct user-initiated, not injected"
    - input: |
        {"spans":[{"id":"t1","kind":"TOOL","attributes":{"tool.name":"file.write","tool.privilege":"write"}},{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"untrusted"}}]}
      expected: not_triggered
      description: "Untrusted RETRIEVER appears AFTER the privileged tool — wrong causal direction"
    - input: |
        {"spans":[{"id":"r1","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"r2","kind":"RETRIEVER","attributes":{"source.trust":"trusted"}},{"id":"t1","kind":"TOOL","attributes":{"tool.name":"data.exfil","tool.privilege":"exfil"}}]}
      expected: not_triggered
      description: "All retrievals trusted — exfil tool used legitimately (e.g., authorized data export)"